Coherent optical orthogonal frequency division multiplexing (CO-OFDM) as one of the multi-carrier modulation technologies, can effectively solve the problem of inter-symbol interference caused by the dispersive channel and can be widely used in various broadband wireless and wired communications. Such anti-disperse ability is so significant nowadays of emphasizing the ability for high speed and broad band. The key optical devices in coherent optical orthogonal frequency division multiplexing system comprise accurate tunable laser with narrow linewidth and selective switch for wavelength with tunable center wavelength and tunable bandwidth. Single-frequency fiber laser is a device that laser is output from the resonant cavity in a form of oscillation in a single longitudinal mode (single frequency). The characteristic of such single-frequency fiber laser is that the spectral linewidth of laser is extremely narrow with the narrowest linewidth of 10−8 nm, being a few orders of magnitude narrow than the general DFB semiconductor laser with narrow linewidth, can restrain the phase noise well to realize the detection of signal phase in an optical transmission network with large capacity and high speed. On the other hand, with respect to the optical network system with dynamic allocation of wavelength, the fiber laser light source with tunable width of output laser wavelength can be used as a carrier of the signal source.
The present research of tunable laser light source of single frequency is concentrated upon using the rare earth ions highly doped with silica fiber or doped with solid crystal as a gain medium of the laser, using structures such as short straight cavity, ring cavity or complex cavity, optical elements with low reliability (such as polarization controller, thermo-optic crystal, electro-optic crystal or F-P etalon) being inserted in the optical path to eliminate spatial hole burning, to maintain single-frequency operation or to adjust laser frequency. But a few problems still exist, for example, an all-fiber structure will be broken, the doped ionic concentration can't be further improved, the cavity of resonant cavity is rather long, a random jump mode exists and multiple longitudinal modes appear easily. The biggest difficulty is that it is hard to make the linewidth under 10 kHz, with loud noise, discontinuous tunable range and poor long-term stability.